The present invention is directed to laminating adhesive compositions suitable for use in various laminating applications. The adhesives are prepared and employed in emulsion form and on removal of the aqueous medium and organic solvent, if any, subsequent to application, the adhesives will cure or harden at room temperature to form a flexible laminate having high bond strength, heat resistance, and a high degree of both humidity and water resistance.
The adhesives are useful to provide laminae of woven and non-woven fabrics where the fabric itself is of cotton, polyolefin, polyester, polyamide (nylon), etc.; coated and uncoated paper and paperboard; films such, for example, as polyvinylidene chloride (PVDC), polyester, PVDC coated polyester, oriented and non-orientated polyethylene and polypropylene film, metallic foils and metallized films; and flexible cellular material such, for example, as polyurethane foams or sponge rubber. The laminates can be made of similar or dissimilar laminae and are useful in a wide variety of end-use applications including, for example, flexible packaging, graphic arts and industrial uses such, for example, as weather stripping and electrical insulation.
The packaging industry, particularly the food packaging area thereof, is currently utilizing large quantities of flexible films. Since all the properties desired in such laminates are not available in any one specific film, the industry generally employes laminates prepared from a combination of films. Very often these laminates are formed from a film of polyethylene terephthalate (PET), polyamide or cellophane, either uncoated or coated with PVDC laminated to a heat sealable polyolefin film which has been treated by corona discharge for adhesion promotion.
In the prior art, the most satisfactory laminates as indicated by industry acceptance have been formed with organic solvent based urethane or polyester adhesives. Most of these adhesives, however, have the disadvantage of requiring organic solvents such as methyl ethyl ketone, ethyl acetate or alcohol in order to form an applicable solution. Due to the desirability of eliminating solvents from such adhesives because of their increasing cost, flammability, as well as pollution considerations, the development of an aqueous emulsion adhesive system capable of performing comparably to the solvent adhesives becomes vital to the continued growth of the industry.
Water-borne laminating adhesives have been described in the prior art. For example, U.S. Pat. No. 3,905,931 issued Sept. 16, 1975, describes one-part adhesives using an aqueous emulsion of a poly(ethyl acrylate), an ethylene-acrylic acid copolymer and a 1,2-epoxy resin Other water-borne, two-part laminating adhesives are known and have been described as based on an aqueous dispersion of (a) a copolymer of a vinyl ester and/or an acrylic acid ester and/or further copolymerizable monomer(s), (b) an epoxy resin, and (c) an amine hardener catalyst.
The adhesives noted above employing an amine hardener catalyst are characterized as two-part adhesives, i.e., they are sold in two parts and must be combined in specified amounts by the user prior to their use in the laminating process. The packaging and selling of these adhesives in two parts (a component comprising an epoxy resin or epoxy resin and vinyl polymer and separate amine catalyst component) is necessary because of the inherent reactivity and instability of the adhesive when the two components are combined. Typically a completed adhesive where the components have been combined exhibits a pot-life of less than 24 hours.
Because of the many inconveniences and disadvantages associated with the two-component adhesives, there is considerable industry interest in stable one-part, ready-for-use laminating adhesives which eliminate formulation and pot-life problems. While aqueous dispersion, one-part adhesives lacking epoxy resin are known and currently sold to the industry, laminations produced with these products have little or no water resistance and very low humidity resistance.
Accordingly, there is still a need in industry for a water-borne, one-part laminating adhesive which in use exhibits bond strength, water and humidity resistance equal to the current two-part adhesives.